1. Field of the Invention
This invention relates to the thermal toughening of glass and more particularly to a method and apparatus for the thermal toughening of a glass sheet in which the sheet is heated to a temperature near to its softening point and the hot sheet is lowered into a gas-fluidised bed of particulate material so that heat exchange between the surfaces of the glass sheet and the fluidised material induces toughening stresses in the glass.
2. Description of the Prior Art
The fluidised bed toughening process has been found to be effective with all glass thicknesses of practical interest, but in the case of thinner glass sheets a difficulty can arise in that the glass sheets may fracture on immersion in the fluidised bed of particulate material.
It is beleived that one important cause of fracture of thin glass sheets in the fluidised bed toughening process is the fact that the leading edge of the glass sheet cools at too rapid a rate as compared with the remainder of the glass sheet. Such a rapid of cooling of the leading edge of the glass puts the leading edge into a high degree of tension. Since glass stressed in tension is weak the glass sheet readily fractures and the fracture generally propagates from any finishing flaws which may be present in the leading edge of the glass sheet.
The incidence of such fractures can be reduced, or avoided completely, by producing a high quality finish on the leading edge of the sheets so as to eliminate flaws. However the necessary edge finish is expensive to produce.
The incidence of fracture of thin glass sheets in the fluidised bed toughening process can also be reduced by heating the sheets, before quenching, to a temperature which is so high that the leading edge of each glass sheet is sufficiently plastic to withstand the transient tensile stresses which arise in the leading edge of the glass sheet when it is immersed in the fluidised bed. However the glass sheets have to be heated to a temperature higher than is necessary to achieve the required degree of toughening and at this higher temperature thin glass sheets are sufficiently plastic to be liable to overall shape distortion during the toughening process.
It is an object of the present invention to minimise the problem of glass fracture in the fluidised bed toughening process so as to obtain a reduced incidence of fracture of thin glass sheets in particular; or in the case of a process in which the incidence of fracture is acceptable to enable the process to be operated using glass sheets having a poorer quality and less expensive edge finish; or to enable processing of the glass sheets at a lower temperature such that the problem of overall shape distortion is minimised.